BPF kfuncs and vmlinux.h

[danobi]

Hi,

I noticed in 3f3751c that the "traditional" vmlinux.h was stripped down. I think in the regular BPF helper world that was probably fine, but I think having a stripped vmlinux.h has implications for the new kfunc world.

For example, kfuncs will now expose internal kernel data structures -- I'm mostly thinking of struct xfrm_state and https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=403f3e8fda60f1a3e54741742d46aea98ecf671e. Those structs depend on a lot of internal kernel types. Maintaining all the dependencies by hand could get tricky.

I was wondering if y'all had any thoughts on the path forward for calling kfuncs.

[tohojo]

Not really, other than yeah, tedious :) Tooling to dynamically generate the stripped-down header would be pretty neat (by recursively expanding dependent types from a list of kfunc names, say). Don't think any such tooling exists, though. I believe there was also a plan at some point to have the kfunc signatures themselves included in the generated vmlinux.h? In which case you could just use the full file and then just call the functions without having to make any further definitions...

[danobi]

Not really, other than yeah, tedious :)

Tooling to dynamically generate the stripped-down header would be pretty neat (by recursively expanding dependent types from a list of kfunc names, say). Don't think any such tooling exists, though.

I did something like this for libbpf-rs originally (although much of it has changed since then): https://github.com/libbpf/libbpf-rs/blob/7f3ba404486f7a9b2d02ed5717c81e92e8a3425b/libbpf-cargo/src/gen/btf.rs#L249-L252 . I think it should be fairly straightfoward for bpftool to do this as well. Although I wonder what bpftool folks think about this vs generating entire vmlinux.h. cc @anakryiko @qmonnet

I believe there was also a plan at some point to have the kfunc signatures themselves included in the generated vmlinux.h? In which case you could just use the full file and then just call the functions without having to make any further definitions...

Yeah that's something that's been bugging me for a while as well. I found libbpf/bpftool#98 which is kinda related.